Silicone rubber composition containing corrosion inhibiting curing agent



United States Patent O 3,364,161 SILICGNE RUBBER COMPGSITION CONTAININGCORROSIGN INHIBITING CURING AGENT Coleman Nadler, Philadelphia, Pa.,assignor to the United States of America as represented by the Secretaryof the Navy No Drawing. Continuation-impart of application Ser. No.347,957, Feb. 27, 1964. This application Jan. 19, 1967, Ser. No. 610,722

10 Claims. (Cl. 260-18) ABSTRACT OF THE DISQLGSURE The subjectdisclosure relates to an improved corrosion inhibiting curing agentsystem for room temperature vulcanizing silicone polymer sealants thatare used to encapsulate and pot electronic equipment. It contemplatesthe use of an amino alkylalkoxysilane and a submicroscopic pyrogenicsilicon dioxide powder with the room temperature vulcanizing siliconeand its organo metallic salt vulcanizing agent preventing the corrosionof the electrical wirin and circuitry of the metallic salt.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout payment of any royalties thereon or therefor.

This is a continuation-in-part of patent application, Ser. No. 347,957,filed Feb. 27, 1964, now abandoned.

This invention relates to a new and improved process for the sealing ofelectrical equipment with silicone compositions capable of vulcanizingat room temperature to form elastomeric products and to improved curingsystems for sealing such equipment.

Silicone rubber stocks based on siloxane polymers with fillers,vulcanizing agents. cross-linking agents, pigments, catalysts and otheradditions are well known and commercially available. Room temperaturevulcanizing (hereinafter RTV) silicone rubber stocks have an advantageover other types of elastomeric products in that they are in fluid formwhich can be poured in or around objects and which can be converted to asolid elastomeric form at room temperature following addition of acuring or vulcanizing agent. An RTV silicone rubber stock consistsessentially of a linear siloxane nolvmer having a molecular formula R i2SiO(R-,SiO) SiRqR wherein each R is a monovalent hydrocarbon radical,each R is either hydrogen. an alkoxy radical or a hydroxv radicalrouping and x has an average value of from 50 to 10.000.

Orcano-metallic salts from the class consisting of dibutyltin dilanrate,lead octoate and stannous octoate effect a condensation reaction of theR groups in these silicones at room temperature and effect avulcanization eonivalent to that obtained with peroxides and heat. ThisRTV silicone rubber is used in the electrical field for potting orencapsulating aircraft and missile electrical and electronic equi ment.It serves to improve the operational reliabilitv of this equipment byinsulating the conper wires against corrosion and contamination from thevarious fluids present in the ambient environment. These elastomerspossess outstanding electrical insulatin properties and can be used athigh temperatures up to 600 F.

However, it was discovered that the presence of the organo-metallic saltcuring agent in the silicone rubber contributed to the corrosion of thecopper electrical wires being encapsulated. The organic acid portion ofthese curing agents react with the copper wires in the presence ofmoisture to form a copper salt and co per oxide corrosion products whichmay cause a malfunction in the equipment, particularly where smalldiameter wires are involved.

3,364Jdi ice It is one object of this invention to introduce a novelcuring system for providing adequate Working life and also preventingthe corrosion tendency of RTV silicone rubber stocks cured withorgano-metallic curing agents.

Another object of this invention is to provide a novel method forinhibiting corrosion in electrical equipment either encapsulated orpotted with an RTV silicone polymer.

These and further objects and features of the invention will become moreapparent from the following description.

It has become common practice to employ organometallic salts to effect acure at room temperature of hydroxy end-blocked organo polysiloxanepolymers. Unexpectedly, I have discovered that I am able to effect sucha cure to the solid, elastic state by employing a combination ofingredients which retard the corrosive effect of the organo-metallicsalts without sacrificing the working life of the silicone polymer. Themeans whereby this room temperature cure is obtained is due to theconcornitant presence of both an amino-alkylalkoxysilane and asubmicroscopic, pyrogenic silicon dioxide powder which has been preparedin a hot gaseous environment of 1100" C. and which possess an extremelylarge surface area for adsorbing the silane in suflicient quantity. Thissilicon dioxide powder material which is produced under the trade nameof CAB-O-SIL is 99% silica on a moisture free basis, has a particle sizeof .015 micron and has a surface area of 225 m. /g. It was entirelyunexpected and in no way could it have been predicted that the combinedpresence of these two ingredients could alleviate the corrosive activityof the organo-metallic salt curing agent for the reason that attempts toemploy these highly reactive and moisture sensitiveamino-alkylalkoxysilancs in the curing mixes for silicone polymersheretofore caused an acceleration of the curing process whereby a usefulworking life and low viscosity which was practical for the intended useof these polymers could not be obtained. The amino-alkylalkoxysilanesalone, when added to the silicone polymers eiiect a rapid curing actionby virtue of their basisity and ability to condense through theirreactive alkoxy end groups.

I have now discovered that the CAB-O-SIL will cause the adsorption ofthe aminosilane onto its surface and will enable it to be compatible inan organo-metallic salt curing system for RTV silicone polymers.However, when the cured RTV electrical sealant is exposed to a corrosicnproducin environment such as high humidity, the aminoalkylalkoxysilaneis releasedfrom the surface of the silica and reacts with the copperwire to form a protective monomolecular film thereon. Theamino-alkylalkoxvsilane reacts with the acidic portion of theorganometallic curing agent to form an amide or salt. The amide or saltproducts of this reaction also form a protective monomolecular film onthe copper whereby it is protected from corrosive attack.

Suitable for use in this curing system are the aminoalkylalkoxysilanesrepresented by the structural formula:

wherein R" represents an alkyl group such as methyl, ethyl, propyl,butyl and the like, or an aryl group such as phenyl, naphthyl, tolyl andthe like, X represents an alkoxy grou such as methoxy, ethoxy, propoxygroups and the like, R is a divalent saturated aliphatic or saturated orunsaturated cyclic hydrocarbon radical, and preferably having a carbonchain of from 3 to 4 carbons, (b) is an integer having a value of from 0to 2 and preferably a value of from 0 to 1, (d) is an integer having avalue of from 1 to 2, and the sum of (d-l-b) is not greater than 2.Illustrative of such amino-alkylalkoxysilanes aregamma-aminopropyltriethoxysilane, gammaaminopropyltripropoxysilane,gamma-aminopropylmethyldiethoxysilane,gamma-aminopropylethyldiethoxysilane, gammaaminopropylphenyldiethoxysilane, delta-aminobutyltriethoxysilane, deltaaminobutylmethyldiethoxysilane, dcltaarninobutylethyldiethoxysilane,delta-aminobutylphenyldiethoxysilane, gamma-aminobutyltriethoxysilane,gamma-aminobutylmethyldiethoxysilane and the like.

In order that those skilled in the art may better understand how thepresent invention may be practiced, the following example is given byway of illustration and not by way of limitation. Parts are by weight.

Gamma-21minopropyltriethoxysilane and CAB-O-SIL (SiO which has beendried for 16 hours at 225 F. are mixed together in a 3:2 weight ratiorespectively. 133 parts by weight of this silicon dioxide-amino silaneester mixture is thoroughly mixed into 600-800 parts by Weight of adimethyl polysiloxane oil which has 250 'parts by weight of copperphthalocyanine powder dispersed therein. 100 parts by weight dibutyltindilaurate are added at the rate of 1 cc./min. dropwise to the mixtureand the entire composition is blended to form a smooth paste. Thepresence of the copper phthalocyanine powder inhibits by an adsorptioneiiect the gamma-aminopropyltriethoxysilane adsorbed on the surface ofthe CAB- O-SIL from reacting with the organic metallic salt andcoagulating the paste. This paste is ready for use and can be exposedand employed over a long period of time (up to one year) so long as itis kept essentially water free.

It is imperative that the dibutyltin dilaurate be added to the dimethylpolysiloxane oil after the gamma-aminopropyltriethoxysilane has beenadsorbed onto the surface of the CAB-O-SIL; otherwise the curing effectof the system will be destroyed. Moreover, in the absence of CAB- O-SIL,the gamma-aminoproyltriethoxysilane would result in a Working time(period during which viscosity of material is still suitable for pottingor encapsulating) of less than 18 minutes. Whereas dibutyltin dilaurateproduces a working time of 160 minutes. However, in the presence ofCAB-O-SIL, this equivalent amount of gamma-aminopropyltriethoxysilanepermits a Working time, of 170 minutes as is noted in the tablehereinunder.

13-26 parts of the paste is mixed with 1000 parts of the RTV siliconepolymer wherein it reaches a Shore 7 A durometer hardness of 50 in 20-24hours at 77 F. The RTV silicone polymer was a hydroxy endblockeddiorganosiloxane having the general formula HOR SiO (R SiO) SiR OH whereR is a methyl radical and x is 250 to 800. The polymer had a viscosityof approximately 50,000 centipoises at C. v

Tests were conducted on RTV silicone polymers cured with the curingcomposition of this invention and compared with an RTV silicone polymercured with dibutyltin dilaurate. Results are given in the followingtable.

Change in Weight, Percent; Volume, Percent;

Shore A Durcmeter Hardness, pts.

Clean and shinyno corrosion film.

Heavy green-brown Copper Wire Cor- V corrosion film, wire rosion (30days at 05-98% RH, 120 etched. F.).

Oven Oven 7 Volume Resistivity After .Aged After Aged (ohm-cm.) Cure 70hrs./ Cure 70 hrs./ 400 F. 400 F.

At 77 F 2.5)(10 21x10" 9.5)(10 2.5)(10 At 300 F 32x10 25x10 2.0)(162.6X10

After 40 days at RH, 120 F. Volume Resistivity at 77 F 1.4)(10 7.5X10

It will be noted that the RTV silicone polymer cured with thecomposition of Example I possessed superior corrosion resistantproperties to the silicone polymer cured with dibutyltin dilauratealone.

Having thus described my invention and a certain embodiment thereofwhich serves as an illustration and not as a limitation, I would have itunderstood that various changes in the details, materials andarrangement of steps Constituent: Parts by weight Copper phthalocyanine250-500 Dimethyl polysilox-ane oil 4.... 600-800 A mixture consisting ofan aminoalkylalkoxysilane of the general formula wherein R is a divalentsaturated aliphatic or saturated orunsaturated cyclic hydrocarbonradical having a chain of at least 3 carbons and wherein nitrogen isattached to at least a third carbon removed from silicon, R represents ahydrocarbon group selected from the class consisting of methyl, ethyl,propyl, butyl, phenyl, naphthyl, tolyl, X represents an alkoxy groupselected from the class consisting of methoxy, ethoxy and propoxy and(b) is an integer having a value of 1 to 2, (d) is an integer having avalue of from 1 'to 2, and the sum of (d-l-b) is not greater than 2,adsorbed on the surface of pyrogenic silicondioxide powder.

'An organo-metallic salt selected from the group consisting ofdibutyltin dilaurate,

stannous octoate and lead octoate 2. A curing composition according toclaim 1 wherein the arnino-alkylalkoxysilane isgamma-aminopropyltriethoxysilane.

3. A curing composition according to claim 1 wherein the organo-metallicsalt is dibutyltin dilaurate.

4. The cured composition obtained from a mixture of ingredientscomprising (a) a linear, fluid polysilortane, polymer of the generalformula wherein each R is a monovalent hydrocarbon radical, each R isselected from the group consisting of hydrogen, alkoxy radicals andhydroxy radicals and x has an average value of from 50 to 10,000, (b)copper phthalocyanine, (c) dirnethyl polysiloxane oil, (d) anaminoalkylalkoxysilane of the general formula RI!b (H2N-R-) dSiX(l-(dflfi) wherein R is a divalent saturated aliphatic or saturated orunsaturated cyclic hydrocarbon radical having a chain of at least 3carbons and wherein nitrogen is attached to at least a third carbonremoved from silicon, R" represents a hydrocarbon group selected fromthe class consisting of methyl, ethyl, propyl, butyl, phenyl, naphthyl,tolyl, X represents an alkoxy group selected from the class consistingof methoxy, ethoxy and propoxy and (b) is an integer having a value of 1to 2, (d) is an integer having a value of from 1 to 2, and the sum of(d-i-b) is not greater than 2, and aminoalkylalkoxysilane being adsorbedon the surface of pyrogenic silicon dioxide powder, and (e) anorgano-metallic salt selected from the group consisting of dibutyltindilaurate, stannous octoate and lead octoate.

5. A cured composition according to claim 4 wherein theaminoalkylalkoxysilane is gamma aminopropyltriethoxysilane.

6. A cured composition according to claim 4 wherein the organo-metallicsalt is dibutyltin dilaurate.

7. The process for obtaining a composition of matter which can beconverted at room temperature to the cured solid elastic state and whichis non-corrosive to a metal encapsulated therein, Which processcomprises forming a mixture of ingredients comprising (a) a linear,fluid polysiloxane polymer of the general formula wherein each R is amonovalent hydrocarbon radical, each R is selected from the groupconsisting of hydrogen, alkoXy radicals and hydroxy radicals and x hasan average value of from to 10,000, (b) copper phthalocyanine, (c)dimethyl polysiloxane oil, (d) an amino-alkylalkoxysilane of the generalformula Rllb (H2NR) S iX(4 4- d+b) wherein R is a divalent saturatedaliphatic or saturated or unsaturated cyclic hydrocarbon radical havinga chain of at least 3 carbons and wherein nitrogen is attached to atleast a third carbon removed from silicon, R" represents a hydrocarbongroup selected from the class consisting of methyl, ethyl, propyl,butyl, phenyl, naphthyl, tolyl, X represents an alkoxy group selectedfrom the class consisting of methoxy, ethoxy and propoXy and (b) is aninteger having a value of 1 to 2, (d) is an integer having a value offrom 1 to 2, and the sum of (d+b) is not greater than 2, andamino-alkylaL koxysilane being adsorbed on the surface of pyrogenicsilicon dioxide powder, and (e) an organs-metallic salt selected fromthe group consisting of dibutyltin di-laurate, stannous octoate and leadoctoate.

8. The process as in claim 7 in which the aminoalkylalkoxysilane isgamma-amiuopropyltriethoxysilane.

9. The process as in claim 7 in which the organometallic salt isdibutyltin dilaurate.

11 A composite article comprising a copper substrate encapsulated withthe cured composition of claim 4.

References Cited UNITED STATES PATENTS 3,203,969 8/1965 Pines et a1.26046.5 3,247,281 4/1966 Gagliardi 26018 DONALD E. CZAI A, PrimaryExaminer.

M. I. MARQUIS, Assistant Examiner.

